Valve power supply - How to size transformer?

[metalsculptor]

Going a bit OT
The cathode interface phenomenon was new to me so I did some digging It turned out to be a chemical reaction (reduction) between a cathode substrate constituent and the oxide coating, as a result it is an integral of a time temperature function. Meaning that it is mainly dependant on heater on time and temperature. The same factors govern loss of emission.

There may be some electrochemical process as a result of cathode current which might partially inhibit the reaction due to the cathode coating being positive with respect to the substrate during current flow favouring oxidation not reduction.

It is unlikely a short heater preheat for an amplifier is which is going to be run for hours would have much effect. It can be avoided by choice of a suitable cathode substrate or barrier layer, whether or not a particular tube manufacturer does so would have to be gleaned from their application notes. It only affects oxide coated cathodes so 833A users have no need to worry .

[AJT]

So do I give these specs to the transformer supplier or do I just order a thumbsuck like:

5v 4A
6.3v 4.5A
290v 600mA

total secondary loading:
20 + 28.35 + 174 = 222.35va

primary VA = 222.35/0.70 = 317.64 say 350VA.

[SanderW]

Hi guys,

Thanks for all the advice! And one more question to prove my ignorance:

Keeping to round numbers: A secondary 0-100V @ 1A through a solidstate bridge rectifier gives approx 140V DC @ 1A.

And a secondary 100 - 0 - 100 through 2 diodes / valve rectifiers also gives 140V DC @ 1A.

So when calculating the seconday VA on a centre tapped secondary you obviously can't work on a 200V swing @ 1A. Do you just always use half the total voltage for the VA calculations?

Intuitively the power drawn is the same between the two transformers, does this mean that half the voltage is 'wasted'?

And then I assume you can run a 'normal' 0 - 100V transformer with 4 valves to maky up a bridge rectifier, has anyone ever done this?

thanks!
Sander

[DF96]

Originally Posted by metalsculptor

whether or not a particular tube manufacturer does so would have to be gleaned from their application notes.

If the datasheet doesn't mention it you can assume that it has an ordinary cathode.

[metalsculptor]

Originally Posted by SanderW

Hi guys,

Thanks for all the advice! And one more question to prove my ignorance:

So when calculating the seconday VA on a centre tapped secondary you obviously can't work on a 200V swing @ 1A. Do you just always use half the total voltage for the VA calculations?

No there is a utilisation factor which depends on the rectifier and filter setup. A bridge rectifier with a choke input filter has a utilisation factor near 1, so the secondary VA rating is similar to the load power, capacitor input filters have utilisation factor between 1.5 and 1.8, full wave centre tapped rectifiers with choke input filter are around 1.5.

Intuitively the power drawn is the same between the two transformers, does this mean that half the voltage is 'wasted'?

Not quite but it does mean that the secondary needs more copper than with a bridge rectifier because current is only flowing half the time so the winding can be made roughly 0.7 times the area of a winding in which the current flows all the time but there are 2 windings so there is 1.4 times as much copper in the secondary.

And then I assume you can run a 'normal' 0 - 100V transformer with 4 valves to maky up a bridge rectifier, has anyone ever done this?
thanks!
Sander

Yes but the voltage drop across thermionic rectifiers is quite high, even mercury vapour ones have 15V drop, I have seen one mercury vapour bridge rectifier but you need 3 heater windings for a single phase and 4 for a 3 phase, bridge rectifier.

[metalsculptor]

One more thought, If a toroidal HV transformer is available, it is a trivial exercise to wind a few turns of hookup wire around the core to get one or more 5V windings for the rectifier heaters. Saving the expense and bulk of another transformer. Because the primary will be oversized to get the appropriate secondary rating there should be enough surplus capacity to run 20 or so watts of heater.

If you get a custom transformer made then the transformer company will know what size secondary wire to use if you tell them the rectifier type and filter capacitor size. The last time I had a custom transformer made it was reasonably priced YMMV.

[SanderW]

Thanks guys,

I will have a custom transformer made, but I have been playing with adding windings onto toroidals just to test out concepts. Toroids really make it easy to play with, but they don't quite look the part in a valve amplifier!

I know with normal EI transformers good practice is to turn them so that the magnetic fields cut each other at right angles.

But if I have more than one toroidal transformer, can I stack them on top of each other without causing a problem? I assume I can't have a conductive bolt through all of them as that would constitute a one turn winding through all the transformers, but with a non-conductive central shaft this should not be an issue?

[toprepairman]

Max anode dissipation for an 807 (it's just a 6L6 after all), is 25W. That means max dissipation for 4 off will be 100W, at 400V is 250mA max. Frankly its a bit much and I wouldn't run them at more than 20W each so 200 mA for the anodes is tops. Allowing 10 to 20 mA for the screens and a tiny bit for the rest of the circuit means a 250mA capable supply is more than adequate.
Regards
Henry

[emosms]

Can anybody point me to some free source "how to design power suppies for valves".
Ive been looking through some schematics, and it looks like after the rectifier there are resistors in series giving several HT in diminishing order, as each HT "point" is capped to ground.
Usually the voltage goes down as the resistors go on,
the latest valve stage getting the higher voltage.
I see some pattern, but how to calculate this?
View image: power supply
For example, I am going to use cheap transformer scheme (220:6v - 6:220v, 2 transformers and getting the heating in the middle), so I need to calculate my power supply for 220 volts raw.
Also, I newer managed to understand how exactly the voltage changes after diode bridge and capacitors - seems to go higher, so I don't know which voltage to count on - the transformer's raw, or whatever I measure after rectification/capping???
It is best, If I understand how to calculate it

[DF96]

Ohm's law will get you a long way. Also see the valve wizard; his advice is generally reliable.